There is an oft-repeated axiom among much of the folks at various Sudbury
schools I have been totally fortunate to associate with, that a good
guideline for finding that "fuzzy line" between leading a child and walking
alongside is: Is what I'm saying to her something I would say to an adult
casual acquaintance?

In other words, I would not walk up to an adult that I had known for a short
time and say, "Hey, I noticed you working with a calculator yesterday. You
know, there's this great book on mathematics in the library."

I might, however, walk up to a casual acquaintance and say, "Was that xyz I
saw you reading the yesterday? Oh, how do you like it? I read her other
book, zyx." etc.

Knowing and being honest with yourself regarding your intention when you
speak to a student is likewise important. Avoiding the "seduction of the
teachable moment". :)